Polymer products, particularly acrylic products, having a granite appearance and methods for their production are known in the art. Such products have found particular application in the spa and bathware industries.
In general, known methods for the production of acrylic compositions to form acrylic sheet products having a granite appearance comprise the addition of various inorganic or organic particles or fillers to an acrylic solution or syrup. For example, U.S. Pat. Nos. 4,159,301 and 4,085,246 disclose a simulated granite material comprising a matrix of acrylic polymer and various opaque and translucent particles having specific optical densities. U.S. Pat. No. 5,304,592 discloses a simulated mineral article comprising a plastic material suspended within a thermoplastic matrix. The plastic material comprises both a thermoplastic and thermoset plastic and the matrix consists essentially of a themoplastic material. U.S. Pat. No. 5,043,077 discloses a granite-like artificial stone comprising a matrix of radical-polymerizable monomers (containing methacrylate and an aromatic vinyl compound) and an inorganic filler, where the filler comprises radical-polymerizable monomers which may be the same or different than those used in the matrix. U.S. Pat. No. 4,959,401 discloses a composition suitable for making synthetic rock comprising an organic portion comprising one or more polymers and an inorganic filler portion comprising an oxide pigment, kaolin and a binder. However, each of these compositions have demonstrated difficulties in processing and later thermoforming.
In an attempt to advantageously affect the processing and thermoforming characteristics of the compositions suitable for forming granite-like products, U.S. Pat. Nos. 5,242,968 and 5,415,931 provide an acrylic product having a matrix of polymethyl methacrylate containing preformed particles of polymethyl methacrylate. The preformed particles are preferably scrap material and comprise 90% of polymethyl methacrylate and 1% of crosslinker, the remainder of the particles being comonomer.
The use of a polymeric material as the particulate or filler component of such compositions has been found to be preferred over the various fillers previously used. Such particles provide sufficient opacity to the composition for hiding and decorative purposes. However, care must be taken in formulating such compositions since the polymeric particles will swell due to the absorption of the matrix monomer to a volume that may be several times greater than their initial volume. The swelling of the particles will increase the viscosity of the composition and prevent the particles from settling.
Such compositions have been used in practice only with difficulty, as the size and swelling of the particles have not been heretofore optimized. In order to produce a material which is easily prepared, aesthetically pleasing and particularly suitable for further processing, the size and swelling rate of the particles must be controlled and optimized. In conventional methods for preparing granite-appearing acrylic materials, such as that disclosed in the U.S. Pat. Nos.5,243,968 and 5,415,931 patents, the particles are often made from cell cast, continuous cast or extruded sheet material. These particles are often difficult to use as they either swell to an extent or at a rate which does not provide optimum results.
If the particles swell too much, the composition may absorb nearly all of the liquid or matrix phase, making its viscosity so great as to be unworkable. Alternatively, the particles may dissolve, losing their character altogether. If the particles do not swell to a sufficient degree, the mixture may not increase in viscosity efficiently, allowing the particles to settle, thus detracting from the granite aesthetic. On the other hand, if the particles swell too slowly, the manufacturing process must be designed to accommodate a dynamic mixture which is constantly changing in viscosity or sufficient processing time must be allowed to allow the composition to reach equilibrium.